A neodymium,strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust |
| |
Authors: | Malcolm T McCulloch Robert T Gregory GJ Wasserburg Hugh P Taylor |
| |
Institution: | Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 U.S.A. |
| |
Abstract: | In the Samail ophiolite,147Sm-143Nd,87Rb-87Sr, and18O/16O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and18O/16O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the147Sm-143Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial143Nd/144Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in εNd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The87Sr/86Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (εSr = ?19.7 to +30.5) and the δ18O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the18O/16O water-rock fractionation. |
| |
Keywords: | |
本文献已被 ScienceDirect 等数据库收录! |
|